An electrical discharge machine is commonly equipped with a power supply for providing high-frequency machining pulses to the dielectric filled machining gap formed between the tool electrode and the workpiece in a dielectric containing or collecting work vessel. In conventional EDM power supply circuit arrangements and methods, machining power pulses are produced in a unit (power supply unit) provided separately from the machine proper which carries mechanical components and the work vessel in which the machining gap is defined between the tool electrode and the workpiece. The separate power supply unit is adapted to package in its cabinet all principal electrical components required to produce at its output a succession of unidirectional power pulses of a predetermined polarity relative to the tool electrode and the workpiece, the output of the unit being connected and hence the power pulses outgoing from the unit being transmitted to the machining gap by way of an elongated cable or line which must be provided. As a consequence, stray resistance and inductance included in the cable or line impede transmission of the unidirectional electric power pulses and act to cause a considerable loss of power and distortion of pulse waveform transmitted to the machining gap. It has, therefore, been unavoidable for the material removal to be undesirably limited, for the machining efficiency to be unsatisfactorily low and for the entire power unit to become excessively large and bulky in conventional electrical discharge machines.
In the aforementioned copending application, there has been disclosed an improved method of and apparatus for supplying high-frequency electrical discharge machining pulses between a tool electrode and a workpiece across a machining gap flushed with a machining fluid in an electrical discharge machine. The improved method disclosed therein comprises: generating a high-frequency alternating electric power in a high-frequency generator unit disposed remote from the machining gap, transmitting the high-frequency alternating electric power to a region immediately proximal to the machining gap along a feeder having inlet terminals directly leading from the high-frequency generator unit; and rectifying the high-frequency alternating electric power in the proximity of the machining gap to form a succession of pulses of a given polarity relative to the tool electrode and the workpiece, constituting the electrical discharge machining pulses which generally have a frequency of 1 kHz and 5 MHz.
An apparatus or a circuit arrangement for carrying out the method has also be disclosed comprising: unidirectional current conducting means disposed in the proximity of the machining gap and connected to the tool electrode and workpiece via respective conductors of a minimum length; a high-frequency alternating power generator disposed remote from the machining gap for providing a high-frequency alternating power at an output thereof; and a feeder for connecting the generator to the unidirectional current conducting means to allow the electric power produced at the output to be transmitted in the form of the high-frequency alternating electric current to the proximity of the machining gap, the unidirectional current conducting means being adapted to rectify the high-frequency alternating electric power in the proximity of the machining gap to form a succession of pulses of a given polarity relative to the tool electrode and the workpiece, constituting the electrical machining pulses.
The unidirectional current conducting means may be constituted by a half-wave rectifier or full-wave rectifier and the feeder may comprise a feeder line or cable, e.g. a coaxial cable of conventional design adapted to carry high-frequency AC power. Further, a transformer (stepdown) may be provided ahead of the unidirectional current conducting means along the feeder to establish a desired magnitude of the electrical machining pulses applied to the machining gap.
According to the technique disclosed, since the machining power from the generator is transmitted in the form of a high-frequency AC until it reaches the proximity of the machining gap, there is little loss of power and distortion of the voltage or current waveform along the feeder, permitting machining at an increased removal rate with a maximum consumption of electric power which can be transmitted with ease even at a frequency in the megacycle range. In addition, the low loss of power along the feeder renders the generator compact and smaller in size and capacity.
The transformer and the rectifier can be mounted on a portion of the walls of the work vessel or directly on the head, column or bed of the machine carrying the tool electrode or the workpiece. The transformer is advantageously of a small size since the power carried thereby is of a high-frequency. The rectification of the transformed high-frequency by the rectifier in the proximity of the machining gap provides polarized or unidirectional machining power pulses which are applied to the gap without substantial power loss and waveform distortion.
Machining power pulses obtained by rectification of the high-frequency AC power may have an extremely narrow pulse duration .tau. on of 1 to 100 microseconds and a pulse interval .tau. off of 0.1 to 50 microseconds, both of which can be set at the generator and applied to the machininig gap without power loss. A succession of machining pulses with such short pulse on time or duration .tau. on and pulse off-time or interval .tau. off are capable of producing finely divided electrical discharges which prevent the workpiece surface from rough material removal and thus enable precision machining. Furthermore, with a discharge pulse repetitively created with a short interval, the machining gap allows a next discharge to take place before its complete deionization from the previous discharge. This effectively avoids misfiring with each individual pulse so that an enhanced discharge repetition rate is achieved and with each individual discharge contributing to material removal with maximum effectiveness, a high-speed and high-efficiency machining process ensues.
The aforementioned copending application has also pointed out that in order to achieve better machining efficiency and stability, it is desirable to periodically or intermittently interrupt the high-frequency electrical pulses so that the machining discharges at the gap take place in the form of intermittently occurring successions of high-frequency elementary discharges. To this end, the high-frequency generator incorporates, in addition to a high-frequency AC oscillator of an output frequency in the range of 1 kHz to 5 MHz, a pulser designed to provide a series of pulses of relatively long ON time Ton and relatively long OFF time Toff and to recurrently interrup therewith with high-frequency AC output. As a result, the output of the generator furnishes successive trains of high-frequency alternating pulses. The successive trains are transmitted along the feeder to the rectifier proximal to the machining gap and thereby converted to successive trains of unidirectional pulses for application across the machining gap. Elementary pulses which constitute each train here may have a pulse duration .tau. on=1 to 100 .mu.s and a pulse interval .tau. off=0.5 to 50 .mu.s and, together with the frequency thereof, are defined at the high-frequency AC oscillator. When the rectifier is of half-wave rectification, the pulse duration and interval becomes .tau. on=.tau. off and when it is of full-wave, the relationship .tau. on&gt;&gt;.tau. off results. The oscillator may also be adapted to produce an asymmetrical AC which is obtainable, for example, by superimposing a symmetrical AC of the adjustable frequency upon a biasing DC, in which case any desired combinations of the pulse duration and interval having a relation either .tau. on&gt;.tau. off or .tau. on&lt;.tau. off may be provided as required in accordance with a particular electrode material combination and machining conditions.
As a consequence, trains of pulses with each train comprising a series of elementary pulses having a pulse duration .tau. on and interval .tau. off and recurrent at a frequency f are provided at a relatively long ON time Ton and OFF time Toff and recurrent at a frequency F across the machining gap between the tool electrode and the workpiece to permit machining at an increased efficiency. The On time Ton and OFF time Toff may each range between 10 microseconds and 100 milliseconds while the frequency F of the trains may range between 100 Hz and 100 KHz, these train parameters being chosen according to particular machining conditions and settings.
A gap sensor may be provided to monitor the machining state or condition at the gap by responding to one or more of gap variables such as gap voltage, current and impedance on an average basis or an instantaneous basis within each pulse train or between adjacent pulse trains, together with a pulse parameter controller responsive to the gap sensor for controlling one or more of Ton, Toff and F as controllable train parameters. Of course, parameters .tau. on, .tau. off and/or f of elementary pulses within each train may simultaneously be controlled as desired with an additional controller associated with the high-frequency AC oscillator.
The control of pulse train parameters is such as to decrease the ON time Ton and/or to increase the OFF time Toff and, in other words, to decrease the number of elementary pulses within each train and/or to increase the period in which the elementary pulses are interrupted in each recurring cycle of elementary pulse trains, when the gap sensor indicates worsening or degeneration of the machining state requiring a correction or recovery in response to the gap variables. Thus, discharge-produced machining chips, tar and gases accumulated or excessively remaining in the machining gap are effectively carried away during increased OFF times and decreased ON times to maintain the gap at an optimum stage which allows machining discharges to be created at stability with successive unit pulse trains. It is also desirable to control the average machining current by modifying train ON time and/or OFF time to allow diverse machining operations from high-speed to slow machining or in response to change in the machining area across which the tool electrode and the workpiece are juxtaposed. The high-frequency oscillator is permitted to oscillate when and every time the ON signal is incoming at its input from the pulser.